Thermally insulated metal containers, commonly known as thermos or vacuum bottles, are constructed of an inner and an outer metal casings to create a double-walled space therebetween and evacuating the space to create a thermal insulation barrier. A method for making such thermos bottles has been disclosed in a Japanese patent, JPN 60-36766 (Kokoku). According to this patented method, a thermos bottle shown in FIG. 28 consists of an inner casing 1 and an outer casing 2 having a bottom, to create a space 3 therebetween to be evacuated for thermal insulation. A thermos bottle can be made from such a structure by joining a reduced diameter section 4a, of the body section 4 of the outer casing 2, to the mouth section 1a of the inner casing 1. To the bottom opening 5 (shown at the top in FIG. 28) is attached a bottom section 6 to complete the construction of a container assembly of the thermos bottle. There is a stepped section 6a in the center area of the bottom section 6, and there is an evacuation opening 7 in the central region of the stepped section 6a. This opening 7 is brazed shut with a sealing plate 8 after evacuation of the space 3 to seal in the vacuum.
This vacuum sealing operation is performed by inverting the bottle as shown in FIG. 28, placing pieces of brazing material around the stepped section 6a, supporting the sealing plate 8 to provide a space between it and the stepped section 6a, evacuating followed by heating so as to evacuate space 3 and melt the brazing material to allow the sealing plate 8 to drop under its own weight to braze shut the evacuation opening 7.
This method of joining was susceptible to sealing defects produced by the dislocation of brazing material and the sealing plate 8 prior to brazing due to rushing air streams at the evacuation opening 7 through the space (between it and the stepped section 6a) as well as by the uncontrolled flowing of the brazing material around the peripheries of the stepped section 6a during brazing.
In addition, it was difficult to automate the placement of the brazing material properly, necessitating a manual operation. Furthermore, the sealing plate was necessary only to provide sealing of the evacuation opening 7. These were two of the main reasons for increasing the cost of production of metal thermos bottles.